Development of a Multivariate Predictive Model to Estimate Ionized Calcium Concentration from Serum Biochemical Profile Results in Dogs

A Prospective Assessment of the Etiology of Murine Dystocia

Dystocia, a common murine reproductive condition, is classified as either obstructive, a result of fetal factors such as an oversized fetus, or functional, a result of dam factors such as advanced age. Treatment is based on the dam's clinical condition and the underlying etiology, but usually requires euthanasia. A prospective study was conducted to characterize the etiology of murine dystocia to determine if treatment is warranted. The signalment and experimental, clinical, and breeding histories were obtained, and a targeted serum chemistry panel, radiographs, and a gross necropsy were conducted on mice presenting with clinical signs consistent with dystocia. Obstructive dystocia was diagnosed if the pelvic canal width was less than the diameter of the fetal head closest to the cervix or a fetus was lodged in the pelvic canal. Functional dystocia was diagnosed based on clinicopathologic abnormalities. A total of 54 mice were evaluated over 7 mo with 45/54 (83%) confirmed to have dystocia with the remaining 9 (17%) having other reproductive abnormalities. Of the confirmed cases, 27/45 (60%) were C57BL/6 or on a C57BL/6 background, and the average age at presentation was 181 ± 85 d. The number of mice categorized as having an obstructive (n = 16) compared with a functional (n = 11) dystocia was not significantly different than those in which the definitive category could not be ascertained (n = 18). Neither clinical signs nor clinical pathology were significantly different between mice categorized as having an obstructive compared with a functional dystocia. Hunched posture, lethargy, and vaginal discharge were the most common presentation. Azotemia (BUN: 66.6 ± 10.2 mg/dL, mean ± SE), hypoglycemia (96.11 ± 8.5 mg/dL), and hyperglobulinemia (3.13 ± 0.14 mg/dL) were common. Differentiating obstructive from functional dystocia could not be determined cageside with strong confidence.

Association between lactulose administration and hypercalcemia in dogs medically managed for hepatic encephalopathy

Objectives

Identify if lactulose administration is associated with hypercalcemia in dogs medically managed for hepatic encephalopathy due to a portosystemic shunt and measure any magnitude in change of ionized calcium (iCa) or total calcium.

Animals

Seventeen client-owned dogs diagnosed with a portosystemic shunt and medically managed with lactulose.

Procedure

Total calcium and iCa were measured at Time point 1 (T1), before starting medical management; and Time point 2 (T2), the time of a reassessment after > 3 wk of lactulose administration.

Results

There were no statistically significant differences in iCa or total calcium between T1 and T2. Lactulose dose was positively correlated with change in iCa between T1 and T2.

Conclusion and clinical relevance

This study did not identify the development of hypercalcemia in a population of dogs treated with lactulose for the management of hepatic encephalopathy. A possible correlation was present between dose of lactulose and change in iCa, and this may represent a dose-dependent association.

Predictors of blood ionized calcium concentration in sick adult cattle

BACKGROUND

Data on the factors affecting blood ionized calcium concentration (ciCa2+ ) and diagnostic performance of serum total calcium concentration (ctCa) measurements to detect abnormal blood iCa2+ status are lacking in sick adult cattle.

OBJECTIVE

Assess the association of ciCa2+ with venous blood pH, plasma concentrations of chloride (cCl), sodium (cNa), and potassium (cK), and ctCa, and total protein, albumin, and globulin concentrations in sick adult cattle.

ANIMALS

Two-hundred and sixty-five adult cattle (≥1-year-old) with different diseases.

METHODS

Prospective study. Whole blood pH, ciCa2+ , cNa, cK, and cCl were measured using a blood gas and electrolyte analyzer, whereas ctCa, and total protein, and albumin concentrations were determined using an autoanalyzer. The relationship between ciCa2+ and venous blood pH, plasma cCl, cNa, cK, and ctCa, and total protein, albumin, and globulin concentrations was investigated. Sensitivity and specificity were calculated for ctCa for diagnosis of abnormal ciCa2+ .

RESULTS

Sensitivity of ctCa measurements to detect abnormal ciCa2+ was 66.0% whereas specificity of ctCa measurements was 72.3%. Serum total calcium concentration measurements accounted for 42% of adjusted blood ionized calcium (iCa2+ 7.40 ) concentration variance. Plasma cCl, and cK had explanatory power of ciCa2+ 7.40 , accounting for an additional 21% and 9% of the variance, respectively.

CONCLUSIONS AND CLINICAL IMPORTANCE

Serum tCa measurements failed to accurately predict blood iCa2+ status in ill adult cattle. Serum tCa concentrations and plasma cCl were the strongest predictors of ciCa2+ in sick adult cattle.

Comparison of ionised calcium measured using a portable analyser to a reference method in healthy dogs

OBJECTIVES

To compare the ionised calcium measured on a portable analyser (iSTAT, Abbott) to a reference method.

MATERIALS AND METHODS

Blood samples from 39 apparently healthy dogs were analysed in duplicate using a portable analyser and a reference method (Radiometer ABL800 FLEX). Bland-Altman plots and Passing-Bablok regression were used to assess constant and proportional bias between the two instruments. A within-assay percentage coefficient of variation and total error (TE) was calculated for both analysers. The reference interval was calculated for the portable analyser using the robust method with confidence interval bootstrapping.

RESULTS

The Bland-Altman plot showed a -0.036 mmol/L difference between the two instruments (95% confidence limit -0.08 to 0.01 mmol/L; limits of agreement -0.07 to 0.006 mmol/L). Neither the Bland-Altman plot nor the Passing-Bablock regression (slope -0.03; 95% confidence interval -0.08 to 0.19 and intercept 1; 95% confidence interval 0.83 to 1.2) showed significant proportional bias. The coefficient of variation for the portable analyser was 1.08%, compared to 0.78% for the reference method with a total error of 3.5% for the portable analyser. The estimated population-based reference interval for ionised calcium using the portable analyser is 1.23 to 1.42 mmol/L.

CLINICAL SIGNIFICANCE

For the healthy dogs in this study, compared to the reference method, the portable analyser showed no significant bias for measurement of ionised calcium. Further studies including hyper and hypocalcaemic dogs are required to determine clinical impact of the use of this analyser.

Sugarcane yeast with urea in replace of soybean meal in the diet of Murrah buffaloes does not cause metabolic, endocrine, and mineral disorders

The aim of this study was to evaluate the effect of the replacement of soybean meal by increasing levels of sugarcane yeast and urea on the energy, enzymatic, protein, mineral, and hormonal profiles of buffalo heifers. Twenty Murrah heifers with an average body weight of 168 ± 2.0 kg and 1 year old were used in a completely randomized design. The treatments consisted of the replacement of soybean meal by increasing levels of sugarcane yeast and urea (0, 33, 67, and 100% dry matter). The animals were weighed at the beginning and end of the experimental period, and blood was collected on the final day. The replacement of soybean meal with sugarcane yeast influenced the performance and metabolic dynamics, with total weight gain (P = 0.005) and average daily gain (P = 0.015), presenting, for both variables, a quadratic effect. Considering the biochemical profile, there was an influence on the serum concentration of triglycerides (P = 0.055), the serum activities of aspartate aminotransferase, gamma-glutamyl transferase, and alkaline phosphatase (P < 0.05), in addition to the serum concentration of phosphorus (P = 0.007) and potassium (P = 0.053), which showed positive linear effects. The hormonal profile was not influenced by diets (P > 0.05). The elements copper, iron, and zinc were able to be quantified at all levels of sugarcane yeast and urea in the diets offered. The use of sugarcane yeast with urea promotes adequate metabolic response of buffalo heifers and can replace up to 100% of soybean meal in the concentrate.

Relationship, difference, and diagnostic discordance between blood ionized and total calcium concentrations in client-owned chelonians

OBJECTIVE

To determine (1) the relationship, (2) the difference, and (3) the diagnostic discordance between blood total calcium concentration (tCa) and ionized calcium concentration (iCa) in a population of client-owned chelonian patients.

ANIMALS

161 consecutively admitted client-owned chelonians.

PROCEDURES

Results for tCa, iCa, and other variables were extracted from records. Bound calcium concentration (tCa - iCa) was calculated. General linear models were developed to evaluate factors affecting tCa, iCa and bound calcium concentrations. Diagnostic discordance between tCa and iCa was assessed.

RESULTS

iCa decreased with increasing albumin concentration, it was not influenced by sex, and it was lower in chelonians with disorders of the reproductive tract than in those with disorders of the endocrine/hemopoietic and urinary systems. Total calcium and bound calcium concentrations increased with increasing albumin concentration; they were higher in females and in chelonians with disorders of the reproductive tract. Of the 161 chelonians, 93 (57.7%; 95% CI, 49.7% to 65.5%) would have had a different diagnosis of calcium status based on tCa and iCa results (ie, diagnostic discordance). A 2.2 mmol/L cutoff value for bound calcium could differentiate chelonians with and without disorders of the reproductive tract with a sensitivity (95% CI) of 81.8% (64.5% to 93.0%) and specificity of 76.4% (68.0% to 83.5%).

CLINICAL RELEVANCE

tCa and iCa were related and were associated with albumin concentrations in chelonians. Evaluation of tCa or iCa alone is likely to result in underdiagnosis of alterations of the calcium status. Calculation of bound calcium concentration could help identify chelonians with reproductive disorders.

Prediction of ionized calcium concentration based on total calcium and protein levels in cattle and sheep

Despite being important, there are no equations for prediction of ionized calcium (iCa) in sheep and cattle. The objectives of this study were i) to create equations for the calculation of serum iCa concentration based on the serum concentrations of total calcium (tCa), albumin (Alb) and total proteins (TP) and ii) to investigate whether predicted serum iCa values are beneficial in clinical practice. Serum samples from 30 sheep and 30 dairy cattle were used. Serum tCa was determined colorimetrically, while serum iCa was determined with an ion selective electrode method. Serum Alb and TP concentration were determined using bromo-cresol green and biuret methods, respectively. Ionized calcium was also calculated based on serum tCa, using regression analysis, and with two equations based on Alb and TP concentration. Bland-Altman plots were plotted to evaluate the agreement between measured and predicted iCa; Passing and Bablok (P - B) regression analysis was used to assess their agreement. The initial equations were corrected using the P - B generated equation and Bland - Altman plots were run to evaluate the level of agreement between measured and predicted iCa using the final equations. Six equations were finally created for cattle and 6 for sheep. The total bias exceeded 10.00% in all of them indicating that they are clinically unacceptable for iCa prediction especially when the predicted result is very close to the cut-off point of < 1.00 mmol L^-1. So, it could be suggested that, when necessary, iCa concentration should be directly determined.

Diagnostic performance of predicted ionized calcium in dogs with total hypercalcemia and total hypocalcemia

BACKGROUND

Abnormal total calcium (tCa) values do not necessarily imply dysregulated ionized calcium.

OBJECTIVES

We aimed to evaluate the diagnostic performance of predicted ionized calcium (piCa) regarding true calcium status in dogs with abnormal tCa.

METHODS

This was a cross-sectional multicenter study. piCa and its prediction interval (PI) were calculated in 114 dogs, from three different hospitals, with either increased (62) or decreased tCa (52), All dogs also had ionized calcium and a biochemical profile available. The sensitivity, specificity, predictive values, and diagnostic discordance of piCa to confirm ionized hypercalcemia (iHyperCa) and ionized hypocalcemia (iHypoCa) were calculated using logistic regression analysis.

RESULTS

iHyperCa was found in 28% and 66% of hyperphosphatemic and non-hyperphosphatemic dogs with tCa above the reference interval upper limit, respectively. The piCa correctly classified dogs with iHyperCa in 72.2% of those with hyperphosphatemia and 93.2% of those without hyperphosphatemia. Comparatively, elevating the tCa threshold to 12 mg/dL properly classified dogs 50% and 75% of the time in hyperphosphatemic and non-hyperphosphatemic dogs, respectively. iHypoCa was found in only 31/52 (60%) dogs with decreased tCa. The piCa correctly classified 55.2 to 100% of dogs with iHypoCa depending on the hospital. The PI demonstrated high sensitivity for iHyperCa (100%) and high specificity for both iHyperCa (100%) and iHypoCa (100%).

CONCLUSIONS

Evaluating tCa alone does not reliably determine ionized calcium status. Even with hyperphosphatemia, piCa and its PI represent a reliable alternative to interpret abnormal tCa values when ionized calcium measurements are not available. However, if the tCa reference interval is notably different from 7.6 to 11.4 mg/dL, piCa values might be under/overestimated.

Predicting ionized hypocalcemia: External validation of an ionized calcium prediction model in patients with COVID-19 and renal failure

Background

Ionized hypocalcemia is common in critically ill patients with COVID-19 and is associated with adverse outcomes. We previously developed a linear model that estimates ionized calcium (I_Ca) by adjusting total calcium (T_Ca) for the three components of the anion gap and albumin. On internal validation, it outperformed the popular method that corrects T_Ca for albumin alone (cT_Ca) in diagnosing low I_Ca. In this study, we sought to externally validate our I_Ca model in hospitalized COVID-19 positive patients.

Methods

We retrospectively studied all 200 patients with COVID-19 who were admitted to the State University of New York Downstate Medical Center between March 11^th and April 30^th 2020 and referred to the nephrology service for renal failure, and who had I_Ca measured on a venous blood gas within 25 min of a comprehensive metabolic panel. We compared the performance of the I_Ca model and cT_Ca in diagnosing low I_Ca by ROC analysis, and also examined the accuracy of the absolute values predicted by the two methods relative to measured I_Ca.

Results

On ROC analysis, the I_Ca model was better than cT_Ca (area under ROC curve: 0.872 [0.025] vs. 0.835 [0.028]; p = 0.045). The I_Ca model estimated I_Ca accurately, but the cT_Ca method seemed to overcorrect T_Ca, as a substantial number of patients with clearly normal cT_Ca values had low I_Ca.

Conclusions

In an external validation cohort, the I_Ca model estimated I_Ca accurately and was better than cT_Ca in the diagnosis of low I_Ca. This finding can be useful in guiding direct I_Ca testing.

Predicting Ionized Hypocalcemia in Critical Care: An Improved Method Based on the Anion Gap

BACKGROUND

Low ionized calcium (ICa) is prevalent in critical care patients. It is poorly detected by the popular indirect method, which corrects serum total calcium (TCa) for change in albumin. That correction (cTCa) ignores any concomitant change in the anion-complexed fraction of TCa. We tested whether the diagnosis of low ICa can be improved by further correcting for calcium complexation, represented by the anion gap (AG) or its components-sodium, chloride, and total carbon dioxide (tCO2).

METHODS

We retrospectively studied all patients in our intensive care units between 2009 and 2011 with ICa measured on arterial (n = 310) or venous (n = 462) gas panels within 19 min of a comprehensive chemistry panel. Logistic models to predict low ICa and linear models to estimate ICa were derived in the arterial group and validated in the venous group, using either AG (AG model) or its components (Ion model) as predictors, adjusted for TCa and albumin.

RESULTS

AG and its set of components were each highly significant independent predictors of low ICa. On validation, the logistic Ion model was better than the logistic AG model (ROC curve area ± SE: 0.92 ± 0.02 vs 0.89 ± 0.02; P = 0.008), which, in turn, was far better than cTCa (0.81 ± 0.03; P = 0.0006); the hypocalcemia rates predicted by the models showed good fit with the observed rates. Linear estimates of ICa were too imprecise for clinical use.

CONCLUSIONS

The adjustment of TCa for AG or for sodium, chloride, and tCO2 markedly improves the diagnosis of low ICa. This finding may be useful in guiding ICa testing.

Changes in the Relationship between Ionized and Total Calcium in Clinically Healthy Dairy Cows in the Period around Calving

Simple Summary

Hypocalcemia is a widespread problem in dairy cows in the first days after calving, which increases the risk for secondary diseases. In practice, the measurement of total blood serum or plasma calcium is widely used to diagnose hypocalcemia. The present study demonstrates a high discrepancy between total and ionized calcium specifically around calving, suggesting that only ionized calcium provides an accurate indication of the animal’s calcium status during that period. We developed an optimized model for prediction of ionized calcium from total calcium, non-esterified fatty acids, beta-hydroxybutyric acid, cholesterol, and phosphorous. However, the precision of that model is still unsatisfactory.

Abstract

We aimed to establish a model for prediction of iCa from tCa, using multivariable regressions with diverse blood constituents. Blood was taken from 14 cows at days −2, 0, 2, 4, 7, and 14 relative to parturition. Cows were clinically healthy, and no hypocalcaemia prophylaxis and treatment were applied. Total calcium and further parameters were determined from frozen serum. Ionized calcium, blood gases, and electrolytes were determined from heparin-stabilized blood samples. Linear regression between iCa and tCa was estimated. Precision improved only slightly using a multivariable model. Best precision was achieved when estimating the iCa:tCa ratio from other blood constituents. To identify the reason behind the poorly predictive value of tCa for iCa, the relative changes of iCa and tCa around calving were calibrated to the respective values of day −2 (=100%) for each cow. An increase in the iCa:tCa ratio was observed from 0.43 at day −2 to 0.48 at day 0, followed by a gradual decrease towards 0.43 at day 7. We conclude that routine measurement of iCa should be implemented in the diagnosis of hypocalcaemia. An optimized estimate of iCa from tCa with non-esterified fatty acids (NEFA), beta-hydroxybutyric acid, cholesterol, and phosphorous as co-predictors is still poorly satisfying.

Effect of pH and storage conditions on measured ionised calcium concentration in dogs and cats

BACKGROUND

Aerobic blood sample collection and processing results in increased serum pH and decreased ionised calcium (iCa) concentration. This prospective study aimed to determine the effect of pH and storage conditions on measured iCa concentration in serum samples obtained from dogs and cats and establish correction formulas for use in samples obtained aerobically.

METHODS

Blood samples were collected from 44 dogs and 25 cats; iCa and pH were measured immediately under anaerobic conditions and in samples stored under several aerobic conditions.

RESULTS

Measured iCa concentrations were significantly lower in samples stored at all aerobic conditions than in samples handled anaerobically in both dogs and cats (P<0.01 for all). The largest and most clinically significant differences were noted in samples stored at -20°C for 30 days in both dogs (0.48 mmol/l; 95 per cent CI 0.40 to 0.55) and cats (0.40 mmol/l; 95 per cent CI 0.33 to 0.47). Correction formulas (corrected iCa=measured iCa+coefficient × (measured pH-7.41); coefficient=0.597 for dogs, 0.627 for cats) yielded good agreement between the corrected and the actual iCa concentrations.

CONCLUSIONS

Samples for iCa measurement can be stored at either 4°C or -20°C for 24 hours. Storage at -80°C is recommended for longer storage time periods.

The multivariate predictive model to estimate ionized calcium concentration from serum biochemical results in dogs: External validation

BACKGROUND

Predicted ionized calcium (piCa) can be calculated from routine biochemistry variables using a recently developed predictive model in dogs. However, it has not been evaluated with variables measured from multiple laboratories.

OBJECTIVES

We aimed to (a) externally validate piCa in dogs where biochemistry results were obtained from different analyzers, and (b) compare the diagnostic performances of piCa and total calcium (tCa).

METHODS

A cross-sectional multicentric study on 138 dogs from three different hospitals was performed. The sensitivity (Sen), specificity (Spe), positive (PPV) and negative predictive values (NPV), and diagnostic discordance of piCa and tCa were calculated using logistic regression for ionized hypercalcemia and hypocalcemia. Diagnostic performance fluctuations across hospitals were also assessed.

RESULTS

For ionized hypercalcemia, the Sen (81.8%), Spe (96.1%), PPV (69.2%), NPV (97.7%), and diagnostic discordance (5.1%) of piCa were not significantly different among hospitals or from those of tCa. For ionized hypocalcemia, the Sen (range: 9.7%-53.8%) and Spe (range: 95.6%-99.6%) of piCa and tCa (Sen range: 16.2%-87.8%; Spe range: 58.3%-98.1%) varied across hospitals, although to a lesser extent for piCa. The diagnostic discordances of piCa (20.3%) and tCa (25.4%) were close. The prediction interval (PI) of piCa demonstrated high Sen to screen for ionized hypercalcemia (100%) and hypocalcemia (range: 75%-93.3%), and high Spe to diagnose ionized hypercalcemia and hypocalcemia (100% for both).

CONCLUSIONS

These results support the external validation of piCa in dogs. Its PI represents a notable advantage over tCa to help clinicians explore calcium-related disorders when ionized calcium cannot be readily measured.

A plasma calcium-phosphorus product can be used to predict the lifespan of dogs with chronic kidney disease

A serum calcium-phosphorus (sCaPP) product was assessed for prediction of survival in dogs affected with chronic kidney disease (CKD). Dogs (N = 150) were retrospectively studied and followed up to determine their lifespan using 25 healthy dogs as controls. Blood and urine analyses were performed and blood pressure was measured. The dogs were divided into groups according to sCaPP (higher or lower than 70 mg²/dL²) and International Renal Interest Society (IRIS) stage (IRIS 1-4). Shorter survival was observed with sCaPP > 70 mg²/dL² compared to dogs with sCaPP < 70 mg²/dL² [45.48 days (range: 5.8 to 149 days) versus 505.40 days (range: 113.31 to 539.52 days), mean (95% confidence interval); P ≤ 0.001 respectively]. Similarly, dogs with advanced IRIS stages showed higher levels of sCaPP [mean (95% confidence interval) in mg²/dL²; IRIS 1: 42.83 (range: 29.58 to 62.10); IRIS 2: 63.18 (range: 46.34 to 90.09); IRIS 3: 95.57 (range: 88.34 to 127.19); IRIS 4: 130.38 (range: 125.16 to 153.52)], accompanied by lower survival rates. Therefore, sCaPP could represent a valuable tool in the prognosis of canine CKD.

Determination of a serum total calcium concentration threshold for accurate prediction of ionized hypercalcemia in dogs with and without hyperphosphatemia

Background

Total serum calcium (tCa) concentrations are poorly predictive of ionized calcium (iCa) status in dogs.

Hypothesis

There is an optimal threshold of tCa concentration that is highly predictive of ionized hypercalcemia and this threshold is higher in hyperphosphatemic dogs as compared to nonhyperphosphatemic dogs.

Animals

Nonhyperphosphatemic (n = 1593) and hyperphosphatemic (n = 250) adult dogs.

Methods

Retrospective medical record review of paired tCa and iCa concentration measurements in dogs presented to a university teaching hospital over a 5‐year period. Positive and negative predictive values, sensitivity, and specificity were calculated for tCa concentration thresholds of 11.0‐15.0 mg/dL (upper limit of laboratory reference interval = 11.5 mg/dL) in nonhyperphosphatemic and hyperphosphatemic groups.

Results

In nonhyperphosphatemic dogs, an optimal tCa concentration threshold of 12.0 mg/dL resulted in a positive predictive value of 93% (95% confidence interval [CI], 84%‐98%) and sensitivity of 52% (95% CI, 43%‐61%) for ionized hypercalcemia. An optimal tCa concentration threshold was not identified for hyperphosphatemic dogs. The nonhyperphosphatemic dogs had a higher prevalence of ionized hypercalcemia than the hyperphosphatemic dogs (7 versus 3%, P = .04) and a lower prevalence of ionized hypocalcemia (23 versus 62%, respectively; P < .001).

Conclusions and Clinical Importance

High tCa concentrations are strongly predictive of ionized hypercalcemia in nonhyperphosphatemic adult dogs and should prompt further diagnostic testing to determine the underlying cause of hypercalcemia. In this population, dogs without increased tCa concentrations rarely had ionized hypercalcemia, but iCa concentrations still should be evaluated in patients with tCa concentrations within the reference interval if there is clinical suspicion for calcium abnormalities.

Development and validation of a multivariate predictive model to estimate serum ionized calcium concentration from serum biochemical profile results in cats

BACKGROUND

Measurement of serum ionized calcium is not always available in practice. Total calcium (tCa) might not be reliable for determination of calcium status in cats.

OBJECTIVES

To predict serum ionized calcium concentration from signalment, biochemistry profile and T4, and compare predicted ionized calcium (piCa) to tCa.

ANIMALS

A total of 1701 cats from two hospitals.

METHODS

Cross-sectional study. Cats with serum ionized calcium, biochemistry profile and T4 available were screened over 6 years and included in the training set (569 cats) to create a multivariate adaptive regression splines model to calculate piCa. Diagnostic performances of tCa and piCa and its prediction interval (PI) were compared in 652 cats from the same institution (test set) and 480 cats from a different hospital (external set).

RESULTS

The final model included tCa, chloride, albumin, cholesterol, creatinine, BUN, body condition score, GGT, age, and potassium. For hypercalcemia, piCa was highly specific (test set: 99.8%; confidence interval [CI]: 99.5-100; external set: 97%; CI: 95.3-98.7) but poorly sensitive (test set: 30.4%; CI: 18.3-42.4; external set: 42.5%; CI: 31.7-53.3). For hypocalcemia, piCa was also highly specific (test set: 81.6%; CI: 78-85; external set: 99.6%; CI: 99-100) but poorly sensitive (test set: 57.6%; CI: 50.6-64.6; external set: 0%). These diagnostic performances were comparable to those of tCa. The upper and lower limits of piCa PI had high sensitivity for detecting ionized hypercalcemia and hypocalcemia, respectively.

CONCLUSIONS AND CLINICAL IMPORTANCE

Predicted ionized calcium is useful to confirm suspected hypercalcemia in cats and screen for hypercalcemia and hypocalcemia.

Effects of pH and the plasma or serum concentrations of total calcium, chloride, magnesium, l-lactate, and albumin on the plasma ionized calcium concentration in calves

BACKGROUND

The plasma ionized calcium concentration (cCa2+ ) represents the biologically active form of calcium and is the preferred method for evaluating calcium status in animals. Different pH-corrective equations have been developed for human plasma, but the validity of the equations for bovine plasma is unknown.

HYPOTHESIS

We hypothesized that pH-corrective equations for bovine plasma would be similar to those used for human plasma; cCa2+ was dependent on the plasma concentrations of total calcium (cTCa), chloride (cCl), L-lactate (cLactate), and albumin (cAlbumin); and the in vitro and in vivo cCa2+ -pH relationships would differ.

ANIMALS

Ten healthy calves (in vitro study), 1426 critically ill calves.

METHODS

The in vitro plasma log10 (cCa2+ )-pH relationship was determined by CO2 tonometry of 465 plasma samples. Plasma cCl was altered by equivolume dilution of plasma with 3 electrolyte solutions of different cCl. The in vivo plasma cCa2+ -pH relationship was investigated and validated using clinicopathologic data extracted from the medical records of 950 (model development) and 476 (model validation) critically ill calves.

RESULTS

pH-corrective equations for bovine plasma were similar to those used for human plasma. Plasma cCa2+ increased in vitro with increases in plasma cCl. Plasma cCa2+ in critically ill calves was associated with plasma cTCa, blood pH, plasma cCl, serum cMg, and cL-lactate (R2  = 0.69) but not plasma cAlbumin.

CONCLUSIONS AND CLINICAL IMPORTANCE

Calculation of cCa2+ from cTCa in calf plasma or serum requires adjustment for at least pH and cCl when 1 or both are outside the reference range.